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    <title>Document</title>
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<script>
    //作用域链
    //执行上下文:js运行时的环境
    let n1='hello';
    function fn(){
        let n2=123;
        function fn2(){
            let n3=true;
            console.log(n1);
            console.log(n2);
            console.log(n3);
        }
        fn2();
    }
    fn();
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